DevTools: promote large views with overflow to layers in order to not repaint on scroll.

Source/devtools/front_end/navigatorView.css

 /*
  * Copyright (C) 2006, 2007, 2008 Apple Inc.  All rights reserved.
  * Copyright (C) 2009 Anthony Ricaud <rik@webkit.org>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 84 matching lines @@
 .navigator-tabbed-pane {
     flex: auto;
 }
 
 .navigator-tabbed-pane .tabbed-pane-content {
     overflow: hidden;
 }
 
 .navigator-tabbed-pane .navigator-container {
     overflow: auto;
+    -webkit-transform: translateZ(0);

[caseq, 2014/03/20 09:03:22]

ditto

 }
 
 .navigator-tabbed-pane .navigator {
     padding-left: 0;
+    -webkit-transform: translateZ(0);
 }
 
 .navigator-tabbed-pane .tabbed-pane-header-contents {
     margin-left: 2px;
     margin-right: 28px;
 }